A method of the above mentioned general type is known from the German Patent Laying-Open Document DE 101 06 736 and corresponding U.S. Patent Application Publication 2002-0109581-A1, wherein it is determined whether a redirection or relaying of a contactless data transmission has taken place, by comparing the damping characteristics of the transmission link of the “uplink” and the “downlink” between a first transmitting and receiving unit and a second transmitting and receiving unit. For this purpose, the amplitude value of the transmitted interrogation signal is measured in the second transmitting and receiving unit, and then this value is transmitted back to the first transmitting and receiving unit. Then, in the first transmitting and receiving unit, the value of the amplitude of the interrogation signal that has been transmitted back is compared with the value of the amplitude of the response signal that has been received by the first transmitting and receiving unit. An unacceptable deviation between these two amplitude values in the result of this comparison is an indication that a redirection or relaying of the data transmission has taken place. In actual practice, however, it is a disadvantage that this method does not make it sufficiently difficult to carry out a redirection or relaying using a so-called transceiver, i.e. it remains relatively easy to achieve an undetectable redirection or relaying using such a transceiver.
A further method as background to the present invention is known from the German Patent Laying-Open Document DE 100 05 503 and corresponding U.S. Patent Application Publication 2001-0014117-A1, wherein at least one physical signal value or characteristic parameter of the transmitted electromagnetic wave is reversibly altered. To achieve this, a reply or response signal is altered relative to the interrogation signal, for example with respect to the frequency, in a second transmitting and receiving unit (i.e. a transponder), and is transmitted back to the first transmitting and receiving unit (i.e. a base station). In the first transmitting and receiving unit (i.e. the base station), the frequency of the response signal is altered or changed back again and compared with the frequency of the originally transmitted interrogation signal. If the frequency value determined in this manner lies within a prescribed interval, then it can be essentially completely excluded that a redirection or relaying has occurred.
Yet another method of the general type forming the background of the present invention is known from the German Patent Laying-Open Publication 100 19 277, in which a redirection or relaying is detected by means of a comparison of the transit time of the interrogation and response signals with prescribed values. In other words, a redirection or relaying is positively detected when the measured value of the transit time does not correspond with a prescribed value of the transit time.
Systems for the contactless transmission of data are preferably used in identification systems. Generally, such systems comprise a first transmitting and receiving unit, i.e. a so-called “base station”, and a second transmitting and receiving unit, i.e. a so-called “transponder”. These systems are used for authentication in the motor vehicle field, which represents a main field of application. To achieve a high level of security of the authentication, the distance or range over which the communication can take place is limited to just a few meters for a so-called “passive entry” system, i.e. for unlocking the vehicle doors whereupon the vehicle door can be opened by pulling the door handle. Furthermore, methods are being developed to prevent an unauthorized authentication, especially by means of a redirection or relaying of the communication, and for terminating or interrupting the authentication process in the event such a redirection or relaying is detected.